*****Firstly the Link is to the UK Origin Site****
Using HOLA (Plugin for Chrome) you can browse to Origin as if you are in Mexico.
You can then get the Mexico price of Assassins Creed Unity (Standard Edition) of $39.99 then use the discount code "ORIGIN20" to get 20% off.
This is a link that will take you to the page to install Hola and then will take you to the origin homepage and just search for assassins creed. http://hola.org/access/origin.com/using/vpn-mx
This leaves you with $31.99 price which translates to £20.47 (Using Google Exchange Rates)
For reference on UK Origin it is £49.99 and the cheapest I can find elsewhere in the UK is at Gameseek for £25 but it will probably not get here before Christmas now.